Process of making anhydrite



July'2, 1935. J. E. BooGE ErAL PROCESS oF MAKING ANHYDRITE Filed May18',v 1954 MSQQ uib Qom,

/N VE NTO/75 or w .SULL/VAN, Eoaspr M. Mc /f/NNEY, JAMES 500G:

, r A I 4free/ver or pigment extendenand comprises Patented July 2, 1935UNITED s'mrlszsv PATENT OFFICE The present invention relates to aprocess of making calcium sulfate in the form of exceedmgly smallparticles variety, particularly adapted for use cium sulfate in any anaqueous medium terized by the employment of weakly dehydrating dtemperatures and the addiconditions or elevate tion of a separatelycomposed of or comprising particles of insoluble anyhdrite.

Calcium sulfate is known in four modications:

gypsum, which isla CaSO4.2H2O, plaster the formula CaSO4 andinsolubleanhydrite. is entirely concerned with soluble anhydrite from an tions ofcalcium sulfate.

Chemical analysis composition between the anhydrite modifications. twocompounds, as their n is quite different.l S

uct.

Fig. 2 shows a diffr anhydrite.

Fig. 3 is the X-ra chloride obtained is shown merely purposes.

The bands shown th f1 relative intensity an bands.

The early literature c statements in respect hydrite, the existence oftwo modifications of anhydrous calcium sulfat tection. Careful ch hasshown us that calcination of a 'calcium ,sul-l fate above temperat ydiffraction pattern of sodium e X-ray diffraction patterns from whichthese gures were drawn are r ures by groups of fine thickness of theline I 2,000,342 PRocEss oF Mimmo, ANHYDRITE James E. Booge, Newark, N.J., Robert M. Mc-

Kinney, Linthicum Heights, Md., and RoyW. Sullivan, Richardson Park,Del., assignors to Krebs Pigment & Color Corporation, Newark, N. J., acorporation of Delaware Application May 18, 1934, Serial No. 726,300

31 claims. (c1. ca -122) insoluble anhydrite, whereas dehydration in anaqueous medium, for instance water at 107 C., or very dilute acids atelevated temperature, forms in every instance a very unstable, easilyhydrated of anhydrite of the insoluble as a pigment digesting calof itsother modifications in and is particularly charachydrite type, whichproducts are more or less hygroscopic and easily hydrated and therebydifferentiate from the insoluble anhydrite which is stable againstwater.

For purposes of the present invention we might call those Well knownconditions under which the hygroscopic heini-hydrate and solubleanhydrite are formed mild-dehydrating conditions and use the termstrong-dehydrating conditions only ior such known conditions whereinsoluble anhydrite is formed directly and whenever used herein theseterms should be understood to have this meaning.

We have found that under the mild-dehydrating conditions under whichgypsum is dehydrated in a liquid suspension to hemi-hydrate or tosoluble anhydrite, insoluble anhydrite is formed,A provided the reactionmixture contains a substantial amount of preformed particles ofinsoluble anhydrite which act as a seed.

Diluted sulfuric acid, that is to say of a strength above about 1.5%, isthe most convenient medium for effecting the transformation of gypsum orother form oi calcium sulfate into insoluble anhydrite of ne particlesize according to our invention. Aqueous solutions of salts and acidsother than sulfuric acid can be used but there is in every instancepossibility of side reactions with formation of substances which wouldact as impurities for the anhydrite.

Anhydrite of ne particle size can be pro-v duced according to ourinvention from practically every type of suilciently pure'calciumsulfate, available in the forms oi duohydrate, hemihydrate or solubleanhydrite. We can use natural, ground gypsum; coarsely crystallized,soluble anhydrite; plaster of Paris, etc.; We can rlikewise useby-product calcium sulfate as obtained, for instance, in the manufactureof phosphoric acid from phosphate rock, or gypsum precipitated by theaction of sulfuric acid upon lime or other calcium compounds; or gypsumobtained by the metathetical reaction of a soluble sulfate with asoluble calcium compound;

vThere appears to be no direct relationship between the particle size ofthe calcium sulfate used as the rawmaterial and the particle' size ofthe anhydrite formed in our process, except that it is naturallypreferable to start with a reasonably nely disintegrated calcium sulfateas the speed' prepared seeding substance duo-hydrate of the formulal ofParis, a heini-hydrate of .1/2H2O, soluble anhydrite, The presentinvention the production of inother of the modicaes no diierence insoluble and insolubleY The reactivity of the ame already indicates,anhydrite is a strong contact. with Water it y. Insoluble anhydrite estable in aqueous suspenthe crystal structure of the anhydritemodications is X-ray diiraction patterns.

' evidenc oluble action pattern obtained by action pattern ofv insolubleder the same conditions and a standard for comparison on theoriginalnegativesv of epresented in these figlines and the number and s isintended to indicate d approximate width of the ontains many conflictingto the formation of ane having long escaped deeck of such priorprocesses ures of` about 500 C. forms product either of the hemi-hydrateor soluble anl 'so of reaction, or conversion, extent upon the degree ofmaterial.

We havev found that vided insoluble anhy depends to a certain divisionof the raw many types of finely di'- te form efficient seeds. Forexample, natural insoluble anhydrite after fine grinding is effective;similar, good results are obtained by using a seed obtained by calcininggypsum, plaster of Paris or soluble anhydrite to temperatures at whichinsoluble anhydrite is formed. Other efllcient seeds are produced whengypsum, etc. issubjected to strong dehydrating conditions such astreatment with strong sulfuric acid at an elevated temperature or when asmall amount of lime or other decomposable calcium compoundy is added toa large excess of strong sulfuric acid while maintaining the mixture atan elevated temperature.

` The temperature conditions under which a separately prepared anhydriteseed will transform gypsum or a. hygroscopic calcium sulfate intonon-acicular, insoluble anhydrite depend upon the .acidity of theaqueous medium in which the transformation takes place. In a non-acidmedium, such as for instance a saturated sodium chloride solution, theformation of the anhydrite takes place at temperatures of about 100 C.or

above. In a medium, the acidity of whichcorresponds to 5% sulfuric acid,complete transformation takes place at about or above C.; atconcentrations up to the equivalent of 35% sulfuric acid, transformationinto anhydrite takes place at temperatures as low as 60 C.; still lowertemperatures being effective with increasing acidv concentrations.

The above conditions exemplify mild dehydrating conditions underwhichv,'in the absence of a seed, hemi-hydrate or soluble anhydritewould be formed. I

-It will be evident from the above that there is a wide rangeofconditions under which an anhydrite seed will cause the conversion ofgypsum or a hygroscopic calcium sulfate into insoluble a'nhydrite. Themost practical. conditions exist,

however, when .the aqueous-medium is a sulfuric acid of less than about35% strength, as for instance from 20 to 25% down to 1.5%.

It is unnecessary in the performance of our invention to start with apreformed hydrated calcium lsulfate or soluble anhydrite, it is in manyinstances preferred to form the calcium sulfate in the presence of theseed.

A convenient manner of precipitating calcium sulfate in sulfuric acid isto add lime, CaO, hydrated lime, calcium carbonate, calcium phosphate orother easily decomposable calcium com# pounds to dilute sulfuric acid.Under ordinary conditions gypsum is precipitated. At elevatedtemperature a hydroscopic calcium sulfate, such as henri-hydrate orsoluble anhydrite, is precipitated. If under such mild dehydratingconditions a seed of insolublevanhydrite is added and the digestioncontinued for an hour orV more at elevated temperature in insolubleanhydrite isobtained as the final reaction product.

A very convenient acid liquorv yfrom which to precipitate calciumsulfate or in which the conversion of gypsum intoinsoluble anhydrite canbe effectedis represented by the weak acid liquors obtained by the4hydrolysis Such residual acids contain,

sulfuric acid, with minor amounts of titanium salts Yand usuallysubstantial amounts of iron salts.

' -per litre were slowly run into 484 cc.

the presence of the seed point for one hour of titanium sulfate. forinstance, from lThe impurities, such as iron salts contained therein, donotinterfere with the precipitation and transformation of the calciumsulfate. l

When the formation of calcium sulfate and its conversion into insolubleanhydrite is combined and leffected by a reaction of sulfuric acid witha. decomposable calcium compound, such as lime or calcium carbonate, itis advisable to adjust manner that the final diabout from 1.5% to 10%the reactants in such a. gestion liquor contains free H2804. A

We are illustrating in the following a few applications of our inventionto insoluble anhydrite.

Example I .--Preparation of an in soluble anhydrite seed under strongdehydrating conditions.

277 cc. of a lime slurry containing 5 00g.Ca(OH) n sulfuric acid whichhad previously been heated to about 75" C. Vigorous stirring wasnecessary to prevent spattering. The amount of lime used was such thatthe resulting anhydrite was suspended in an approximately 50% sulfuricacid solution.

` This .precipitate constituted an efficient seed for use in theconversion of gypsum or hygroscopic calcium sulfate compositions intoinsoluble anhydriteunder mild dehydrating conditions.

Seeds prepared in this manner were-found to be very active andamountsdown to about 5% based on the total calcium sulfate to be converted intoinsoluble anhydrite are sufficient, though we generally prefer to uselarger amounts of seed, from l0 to 20% or even more. n

Other eillcient seeds are prepared by calcination of gypsum above about500 C. up to 800 C. or up to temperatures at which calcium sulfatedecomposes.

Natural, ground insoluble anhydrite has also been used very successfullyas a seed in the conversion of gypsum or hygroscopic calicum sulfatecompositions into fine particles ot insoluble anhydrite according to ourinvention.

We have, furthermore, used the insoluble anhydrite produced according toour invention as a seed for subsequent operations, either directly orafter calcination. By re-cycling a part of the anhydrite produced we arein this manner enabled to completely transform gypsum into insolubleanhydrite-using a dilute sulfuric acid as the reaction medium withouthaving to provide for a seed prepared by a different process.

Example -II .-Direct preparation of insoluble anhydrite under milddehydrating conditions in the presence of a seed.

A finely ground, seed was added to cc. o 'f a boiling dilute 25%sulfuric acid, such as obtained as a by-product in the hydrolysis oftitanium sulfate. 833 cc. of Ia lime slurry containing 500 g. Ca(OH)zperlitrewas then run in slowly wlile at the same time 2620 cc. of thesame boiling 25% sulfuric acid'was also run in, care being taken thatthe acid was always in excess. When the precipitation was completed theslurry contained about 20% acid in excess of that neutralized. Afterdigestion at the 'boiling the slurry was filtered hot, washed and dried.The product obtained was very stable against re-hydration and showed anX-ray diffraction pattern identical 'with that of insoluble anhydriteobtained, for instance, by calcining gypsum to temperatures above 500 C.The so obtained insoluble anhydrite is perfectly white and of very fineparticle size. It can be used as a seed in subsequent operation and isperfectly natural, insoluble anhydrite the production of ExampleIII.--Dehydration of gypsum in thepresence of an insoluble anhydriteseed under mild dehydrating con-ditions.

Finely divided gypsum was suspended in a saturated sodium chloridesolution at 100 C.; to this was added 20% of a seed as prepared inExample I and the mass digested for 4 hours at 100 C. The calciumsulfate was substantially completely dehydrated, the product obtainedwas insoluble anhydrite of fine particle size and showed the X-raydiffraction pattern of the insoluble anhydrite.

In the absence of the seed the dehydration of vthe gypsum undery thesame conditions proceeded only to the hemi-hydrated calcium sulfate, asevidenced by chemical and X-ray analysis.

Example IV.--The digestion of gypsum was carrie-d out under conditionsentirely similar to those 'of Example III, except that 40% of a seed wasused which was obtained by disintegration of natural insolubleanhydrite. After 3 hours digestion at C. the product was convertedsubstantially entirely into finely divided particles which showed thecharacteristic X-ray diffraction pattern of insoluble anhydrite.

Example V.Gypsum was suspended in dilute sulfuric acid of 5% strength toform a slurry which Was heated to about 100 C. and 40% based on thegypsum, of natural, ground anhydrite added.- After 3 hours digestion thematerial was substantially completely transformed into a nely dividedcalcium sulfate which showed the characteristic X-ray diifractionpattern of insoluble anhydrite.

Treating gypsum under the same conditions but in the absence of the seedproduced only an insignificant amount of dehydration and no insolubleanhydrite could be found in the digested product. v l

Example VI.-'Io' the seed prepared as in Example I was added in the samevessel 950 grams of finely divided ground natural gypsum and sufficientwater to reduce the acidconcentration to less than 5% HzSOli. 4'Ihe masswas digested at 100 C. for 3 hours, after which the product wasconverted substantially completely into finely divided particles whichshowed the characteristic X-ray pattern of insoluble anhydrite.

Example VII .-2000 grams of soluble anhydrite were suspended in a 5%sulfuric acid to form a stirrable suspension. I'o this was added anamount of insoluble anhydrite seed as ob-.

tained in Example I. The mass was digested for 3 hours at 100 C. Thesoluble anhydrite was substantially completely transformed into theinsoluble modication.

It will be understood that when we speak of a separately prepared seedwe do not limit ourselves to a seed prepared in a vessel different fromthat in which the digestion of thev calcium, sulfate is performed;conditions such as shown in Example VI are `considered to come withinthe meaning of separately prepared seed. The term separately preparedseed is, however, intended to exclude such conditions where the seed andthe calcium sulfate to be converted into insoluble anhydrite are formedjointly by the same chemical reactions. f

Results entirely similar to those described in the above examples areobtained by treatment of plaster of Paris or soluble anhydrite.

Electrolytes present in theaqueous medium used for digesting calciumsulfate have a marked effect on speeding up the conversion of thecalcium sulfate into insoluble anhydrite. The most convenientelectrolyte to be used is sulfuric acid as with this agent there is nodanger of contamination of the reaction product with compounds whichwould be detrimental tothe use of the insoluble anhydrite, good resultsare, however, also obtained in aqueous media containing large amounts ofneutral, soluble salts, such as sodium chloride, sodium sulfate, sodiumnitrate and other alkali metal or ammonium salts Such as potassium saltsand mixtures thereof. Only such compounds can naturally be used which donot react with the calcium and sulfate ion to form compounds lesssoluble than calcium sulfate.

The'temperature of the digestion liquor is conveniently maintainedaround 100 C. or at the boiling point of the conversion media, thoughlower temperatures can be used, provided the conditions selected aresuiiicient to dehydrate gypsum to hemiliydrate or to soluble anhydritein the absence of a seed.

The conditions for the preparation of insoluble anhydrite in thepresence of a seed described in the above batch operations are likewiseapplicable to processes of the continuous type where, for instance, aslurry of a calcium compound and a dilute sulfuric acid aresimultaneously 0 run into a reaction vessel and the reaction product iscontinuously removed. In this case we, for instance, leave a substantialamount of insoluble anhydrite in the reaction Zone or we also addconcurrently a product consisting of insoluble anhydrite. We prefer thento run the overflow into a larger vessel in which the digestion iscontinued for some time before we separate the insoluble anhydrite fromthe reaction liquor. We'can also provide for a suspension of insolubleanhydrite and separately and continuously mix the calcium compound withthe dilute sulfuric acid, or other sulfate and then run the reactionproduct continuouslyeinto the seed suspension.

We claim: Y

l. In a process of making insoluble anhydrite, the step of digestinganother modification of calcium, sulfate in an aqueous medium and atelevated temperature in the presence of aseparately prepared seedcomprising particles of inoluble anhydrite.

2. The process of claim 1 in which the seed is prepared by digestingcalcium sulfate in strong` sulfuric acid under strong dehydratingconditions.

. 3. The process of claim l in which the seed is prepared by mixing limeand strong sulfuric acid under strong dehydrating conditions. l

4. The process of claim l in which the seed is natural ground anhydrite.

5. The process of claim l in 'which the seed is a calcium sulfatecalcined to temperatures at which insoluble anhydrite is formed.

6. The' process of claim l in which the other modification of calciumsulfate is gypsum.

7. In a processof making the insoluble anhydrite modification of calciumsulfate, the step of digesting another of the modifications of caliumsulfate in an aqueous medium under mild dehydrating conditions in thepresence of a separately prepared seed comprising particles of insolubleanhydrite.

8. The process of claim 7 inwhich the seed is prepared by digestingcalcium sulfate in strong sulfuric acid under strong dehydratingconditions.

9. The process of claim 7 in which the seed is prepared separately bymixing lime and strong sulfuric acid vunder strong dehydratingconditions. l

10. The process of claim 7 in which the seed isA natural groundanhydrite.

11. The process of claim 7 in which the seed is prepared by calciningcalcium sulfate to temperatures at which insoluble anhydrite isproduced.

l2. 'I'he process of claim 7 in which theother modification of calciumsulfate is gypsum.

13. In a process of making the insoluble anhydrite modification ofcalcium sulfate, the step of digesting another of the modifications ofcalcium sulfate in an aqueous solution of an electrov lyte, which doesnot react with calcium or sulfate ions to form compounds less solublethan calcium sulfate and in the presence of a separately prepared seed.comprising particles of insoluble anhydrite.

14. In a process of making the insoluble anhydrite modification ofcalcium sulfate, the step of digesting another of the modifications ofcalcium.

sufate in dilute sulfuric acid at elevated temperature and in the'presence' of a separately prepared seed comprising particles ofinsoluble anhydrite. .p

16. The process of claim 15 in which the seed is prepared by digestingcalcium sulfate in strong sulfuric vacid under strong dehydratingconditions. l

17. The process of claim 15 in which the seed is prepared by mixing limeand strong sulfuric acid under strong dehydrating conditions.

`18. The process of claim 15 in which the seed is natural groundanhydrite.

19. The process of claim 15 in which the seed is prepared bycalciningcalcium sulfate to temperatures at which insoluble anhydrite isformed.

20. The process of claim 15 in which the other modification of calciumsulfate is gypsum.

21. In a process of making the insoluble anhydrite modification ofcalciumvsulgte, the step of digesting another of the modifications ofcalcium sulfate in dilute sulfuric acid at elevated temperature in thepresence of a separately prepared seed comprising particles of insolubleanhydrite, the process being further characterized in that the strengthof the acid is between about 1.5 and 35% H2804 and the digestiontemperature is correlated with `the strength of the acid so that for anacid of 35% the temperature is at least 60 C.

and for an acid of 1.5% the temperature is 'at least C. Y

22. The process of claim 21 in which the seed is prepared by digestingcalcium sulfate in strong sulfuric acid under strong dehydratingconditions.

23. The process of claim 21 in which -the seed is prepared by mixinglime and strong sulfuric acid under strong dehydrating conditions.

- 24. The process of claim 21 in which the seed is natural groundanhydrite.

25. 'Ihe process of claim ,21 in which the seed is prepared by calcininga calcium sulfate to temperatures at which an insoluble anhydrite isproduced.

26. The process of claim 2l in which the other modification of calciumsulfate is gypsum.

'27. In a process o'f making the insoluble anhydrite modification ofcalcium sulfate, the step of digesting another modification of calciumsulfate in a waste acid liquor obtained in the hydrolysis of a titaniumsalt solution at elevated temperature and in the presence of a seedcomprising particles of insoluble anhydrite.

28. In a process of making insoluble anhydrite,l

the step of mixing dilute sulfuric acid with a minor amount of insolubleanhydrite and a major amount of another modification of calcium sul"fate and digesting the mixture under mild de` hydrating conditions. 29.The process of making insoluble anhydrite lwhich comprises precipitatingcalcium sulfate from an aqueous solution of an electrolyte at elevatedtemperature and in the presence of a separately prepared seed comprisingparticles of insoluble anhydrite and digesting said mixture undermilddehydrating conditions.

30. The process of making insoluble anhydrite which comprisesprecipitating calcium sulfate from dilute sulfuric acid and digesting itat an elevated temperature in the presence of a separately prepared andadded seed comprising particles of insoluble anhydrite.

31. The process ofclaim 30 in which 4he seed is prepared by digestingcalcium sulfate in strong sulfuric acid under strong dehydratingconditions.

32. The processor claim 30 in which the seed is prepared by mixing limeand strong sulfuric acid under strong dehydrating conditions.

33. The process of claim 30 in which the seed is natural groundanhydrite.

34. The process of claim 30 in which the seed peratures at whichinsoluble anhydrite is produced. 4

35. The process of claim 30 in which the precipitating conditions aresuch that in the absence .of the seed gypsum would be produced.

36. Process of rmaking insoluble anhydrite which comprisesprecipitatingcalcium sulfate in sulfuric acid andadjusting the amount of reactantstoobtain at the end of theprecipitation is prepared by calcining calciumsulfate to tem-l Q an acid liquor of from about 1.5 to '10% freeA H2504and digesting it in the reaction liquor at an elevated temperature inthe presence of a -separately prepared seed comprising particles ofinsoluble anhydrite.

37. The process of making insoluble anhydrite which comprisesprecipitating calcium sulfate from a waste acid liquor obtained in thehydroly-l sis of titanium 'sulfate and digesting it under acidconditions and at elevated temperature in thepresence of a separatelyprepared seed comprising particles of insoluble anhydrite.

JAMES E. BooGE. RoY w. SULLIVAN. ROBERT M. MCKINNEY.

